The present invention relates generally to obtaining a measure of a patient's thoracic impedance, and more particularly to doing so using information derived from an EKG signal with a device that relies on electrical energy provided by the patient's own heart.
Specific resistance of biological materials and impedance measurements have played a major role in modern medicine. The electrical conductivity and capacity of disperse systems have been described as early as 1931 (Fricke et al., The Electric Conductivity of and Capacity of Dispersed Systems; Physics 1931; 1:106-115). Later, especially in the 1950s and 1960s, significant interest was directed towards the resistance of biological materials (e.g., Geddes L. et al., The Specific Resistance of Biological Material: A Compendium of Data for the Biomedical Engineer and Physiologist, Medical and Biological Engineering 1967, 5:271-293). The application of impedance and resistance measurements for cardio-circulatory function by measuring the blood and body temperature has been studied extensively by Geddes et al., Medical and Biological Engineering 1967, 11:336-339). Also, internal and external whole body impedance measurements have been used for noninvasive monitoring and determination of cardiac output (Carter et al., Chest 2004, 125:1431-1 440). In addition, the feasibility of using intracardiac impedance measurements has been evaluated by E. Alt et al. for capture detection in connection with cardiac pacing (Pace 1992, 15: 1873-1 879).
Background patents that describe the use of impedance in conjunction with implantable devices are referenced in U.S. Pat. No. 5,003,976 to Alt, which describes the cardiac and pulmonary physiological analysis via intracardiac measurements with a single sensor. The '976 patent discloses that a single functional parameter, namely intracardiac impedance, varies both with the intrathoracic pressure fluctuations following respirations and with cardiac contraction. This value is representative of both pulmonary activity and cardiac activity. The finding indicates that this information derived from intracardiac impedance can be used not only to monitor the patient's cardiac and pulmonary activity, condition and cardio-circulatory status, but also, to control the variability of the rate of an implantable cardiac pacemaker.
U.S. Pat. No. 4,884,576 to Alt et al. discloses a self-adjusting rate responsive cardiac pacemaker and method based on the intracardiac signal derived from impedance measurements using an electrode implanted into the heart. And U.S. Pat. No. 4,919,136, also to Alt, describes a ventilation controlled pacemaker which uses the ventilation signal derived from those impedance measurements with an electrode in the heart to adjust the pacing rate.
Recently, considerable interest has been focused on the monitoring of congestive heart failure by means of impedance. U.S. Pat. No. 6,473,640 to Erlebacher describes a system that detects changes in resistance to a flow of current in the systemic venous system, and detects changes in impedance to a flow of current through lungs. The specific signal processing enables a determination of congestion in the venous or in the pulmonary system by application of differential signal processing of impedance. Other methods, such as are described by Combs in U.S. Pat. No. 5,957,861 and Riff in U.S. Pat. No. 5,876,353, respectively pertain to impedance monitoring for discerning edema through evaluation of respiratory rate, and use of implantable medical devices for measuring time varying physiological conditions, especially edema, and for responding thereto.
U.S. Pat. No. 6,104,949 to Pitts-Crick relates to a device and a method used for the diagnosis and treatment of congestive heart failure. Godie, in U.S. Pat. No. 6,351,667, describes an apparatus for detecting pericardial effusion, in which a wire probe anchored to the right heart ventricle and two other wire probes are used to measure the impedance between the different probes in order to assess the degree of pericardial effusion.
U.S. Pat. No. 4,899,758 to Finklestein et al. describes a method and apparatus for monitoring and diagnosing hypertension and congestive heart failure. U.S. Pat. No. 6,336,903 to Brody relates to an automatic system and method for diagnosing and monitoring congestive heart failure and the outcomes thereof. US patent publication 2002/0115939 to Moligan et al. describes an implantable medical device for monitoring congestive heart failure in which incremental changes in parameter data over time provide insight to the patient's heart failure state.
The measurement of heart failure becomes of greater clinical interest and importance as more than 5 million patients in the U.S. are affected. With deterioration of myocardial function, patients often require repeated hospitalization. Current methods of monitoring congestive heart failure cannot reliably predict an early occurrence of this congestive heart failure; but an understanding of its occurrence may provide an early indicator of this adverse event for the patient.
A considerable number of new treatment forms have been introduced into clinical practice. It had been shown that congestive heart failure can be treated, not only by drugs, especially Beta blockers, but also by biventricular pacing. This method makes use of the exact timing of a stimulus, not only to the right ventricle or to the septum, but also to the left side of the heart by means of an electrode which is implanted into the coronary venous circulation. By these means, the left ventricle can be stimulated at a time that provides an optimal synchronization of the heart and improves the mechanical effectiveness of the systole by synchronizing the depolarization of the right heart, the septum and the left heart. This avoids the ineffective late contraction of the left ventricle at a time when the septum depolarization has already occurred, and the squeezing of the blood by synchronous action of the septum and left ventricle is no longer present. In addition, the reduction in mitral valve regurgitation by this type of resynchronization has been shown.
Studies published at the 2005 meeting of the American College of Cardiology in Orlando, Fla., USA (CARE-HF study) illustrate that not only the quality of life of those patients with New York Heart Association, Heart Failure Class 3 and 4 can be improved, but also the life expectancy. This recent data show very impressively that over a 3-year period such biventricular stimulation and the mortality can be reduced by half in a highly significant manner. An these new devices improve the survival and quality of life of patients and have a beneficial effect on re-hospitalization. Nevertheless, the occurrence of heart failure is still a major problem for these patients, and it is beneficial to detect such a heart failure as early as practicable.
A second parameter which plays a major role in patients with implantable devices such as pacemakers and defibrillators is the correct adjustment of heart rate. Rate adaptive pacemakers in the past have provided an open type of correlation between a signal parameter to adjust the heart rate and the affected heart rate. However, even multiple sensor parameters that have been used for adjustment of the pacing rate have not brought the real need of a patient to clinical practice, mainly a closed-loop monitoring of heart rate.
In the healthy person, the heart rate is regulated by a very sophisticated closed loop and negative feedback. Heart rate only increases to a level with exercise which is physiologically beneficial. This means that if a patient exercises only mildly, his heart rate increases proportional to the increase in oxygen uptake for this person which is a fraction of his maximum exercise capacity, maximum oxygen uptake and aerobic and anaerobic capacity. Thus, if someone is well-trained; an external load of 50 watts might represent only 25% of his/her maximum exercise capacity if the patient is capable of exercising up to a level of 200 watts. With this external load of 50 watt the heart rate will increase by only the fraction that is represented by the patient's resting heart rate and maximum exercise heart rate. In other words, such a well-trained person will increase his/her heart rate only by 30-35 beats per minute (bpm). A less capable patient, who has a maximum exercise capacity of 100 watts, will increase his/her heart rate with the same external load to a higher degree. In that case, the slope of increase in heart rate depends not only on a fixed relation of a sensor parameter, such as ventilation or physical activity or any other physiologic parameter having a suitable correlation with heart rate, but also on his/her underlying cardio-pulmonary exercise capacity and condition.
It is therefore a principal aim of the present invention to provide a novel method to detect a parameter that can control not only the heart rate in a physiologic appropriate manner, and provide a closed-loop feedback control for the optimal heart rate of an exercising patient, but also to monitor the individual status of the patient under a wide range of physiologic conditions including normal resting status, congestive heart failure and exercise states.
Many attempts have been made in the past to use impedance measurements to derive appropriate signals; however, the past approaches have involved use of external power sources to power the device(s) that would monitor and detect impedance. This external energy can be applied either outside the thorax from a supply external to the body, or by an implantable device that uses energy from an electrical battery housed within the device itself.